1. Field of the Invention
The present invention relates to an isolator utilizing the Faraday effect.
2. Description of the Related Art
FIG. 4 shows a main portion of a conventional isolator. A magnetic core 21 constructed by ferrite of a disc shape such as YIG, etc. is placed within a direct current magnetic field generated from an unillustrated permanent magnet, and its upper face is perpendicular to the direction of the direct current magnetic field. Three central conductors (first central conductor 22, second central conductor 23 and third central conductor 24) are placed on the upper face of the magnetic core 21, and are held so as to be overlapped with each other at an equal angle interval (120xc2x0) by mutually holding their insulations approximately in the central portion of the upper face of the magnetic core 21. Their lengths are approximately equal to each other so that their inductance values are also approximately equal to each other.
Each of the central conductors 22 to 24 is constructed by two parallel band-shaped conductors, and one end of each central conductor is connected to the ground. The other end 22a of the first central conductor 22 is set to an input terminal, and the other end 23a of the second central conductor 23 is set to an output terminal. The other ends 22a, 23a are respectively connected to the ground by capacitors 25, 26 for matching.
On the other hand, the other end 24a of the third central conductor is connected to the ground by a terminal end resistor 27 and a terminal end capacitor 28.
In the above construction, a signal inputted to the other end 22a of the first central conductor 22 is outputted to the other end 23a of the second central conductor 23 by the Faraday effect.
However, a signal of the reverse direction inputted to the other end 23a of the second central conductor 23 is oppositely absorbed by the third central conductor 24 and the terminal end resistor 27 and the terminal end capacitor 28 connected to this third central conductor 24, and is not outputted to the other end of the first central conductor 22. A transmitting state of this signal of the reverse direction will be explained by FIG. 5 constructed by replacing this transmitting state with an equivalent electric circuit.
In FIG. 5, an inductance element 31 connected to the other end 23a of the second central conductor 23 is represented by the third central conductor 23, and an inductance element 32 connected to the other end 22a of the first central conductor 22 is represented by the first central conductor 22. A capacity element 33 between the two inductance elements 31 and 32 is represented by a coupling capacitor between the two central capacitors 22 and 23.
One end of an inductance element 34 represented by the third central conductor 24 is connected to both ends of the capacitor element, and the other end is connected to the ground by the terminal end resistor 27 and the terminal end capacitor 28. Accordingly, a series resonance circuit is constructed by the inductance element 34 and the terminal end capacitor 28, and a signal is greatly attenuated at a resonance frequency F0 as shown in FIG. 3.
However, since a frequency band for attenuating the signal transmitted in the reverse direction is narrow in the above construction, a problem exists in that the attenuating amount R is reduced with respect to the signal transmission of a wide frequency band (from F1 to F2) and the signal level reflected to the input terminal is increased.
An object of the present invention is to provide an isolator for greatly attenuating the signal transmitted in the reverse direction over the wide frequency band.
To solve the above problem, the present invention resides in an isolator comprising a magnetic core of a plate shape placed within a direct current magnetic field and having an upper face perpendicular to the direction of the direct current magnetic field, and first to third central conductors placed so as to be overlapped with each other at an equal angle interval approximately in the central portion of the upper face of the magnetic core, and respectively having one end connected to the ground, wherein the other end of the first central conductor is set to an input terminal, and the other end of the second central conductor is set to an output terminal, and at least the third central conductor is constructed by two parallel band-shaped conductors, and the other end of one band-shaped conductor is connected to the ground by a first resistor and a first capacitor, and the other end of the other band-shaped conductor is connected to the ground by a second resistor and a second capacitor, and the capacity value of the first capacitor and the capacity value of the second capacitor are set to be different from each other.
Further, each of the first central conductor and the second central conductor is constructed by two parallel band-shaped conductors, and the respective distances between the two band-shaped conductors in the first to third central conductors are equally arranged.